Aluminium alloys used for the manufacturing of cylinder heads are generally from the AlSi family with silicon typically ranging from 5 to 10%. In addition to the lowering of the melting point, silicon addition in the aluminium provides the required casting ability, necessary for the manufacturing of parts with ever increasing geometrical complexity. Most widely used casting alloys for cylinder heads belong to 2 main families for which silicon is ranging between 5% and 10% and copper between 0 and 3.5% (depending on the specifications, and using conditions). The first family relates to AlSi7Mg type of alloys (for example A356 in SAE standard) generally T7 heat treated (complete treatment) alloys, well-known for their excellent castability, good damage tolerance and mechanical properties, except at high temperatures. The second family relates to AlSi 5 to 10% Cu3Mg (for example 319 in SAE standard) generally T5 (aging treatment only) alloys, well-known for their economic interest, mechanical resistance at high temperature but poor damage tolerance.
In both cases, the temperature range in which these alloys can be used is limited to 280° C., as their mechanical properties, in particular yield strength, decrease brutally after a few hours (see for example FIG. 1).
From DE 10 2006 059 899 A1 is known a heat resistant aluminium alloy comprising 4.5-7.5 wt % Si, 0.2-0.55 wt % Mg, 0.03-0.50 wt % Zr and/or 0.03-1.5 wt % Hf, maximum 0.20 wt % Ti, <0.3=wt % Fe, <0.5 Mn, 0.1-1.0 wt % Cu, <0.07 wt % Zn, with the rest Al and impurities maximum 0.03 wt %. This reference appears to be concerned with the Cu content to improve the heat resistance of the alloy in combination with relatively large ranges of Zr and/or Hf. The optimum combination is, however not further verified or documented.
US 2006/0115375 relates to a high strength, thermally resistant and ductile cast aluminium alloy comprising 5.5-7.5 wt % Si, 0.20-0.32 wt % Mg, 0.03-0.50 wt % Zr and/or 0.03-1.50 wt % Hf, 0-0.20 wt % Ti, <0.20 wt % Fe, <0.50 wt % Mn, <0.05 wt % Cu and <0.07 wt % Zn. The objective with this known alloy is to retain its strength values at temperatures equal to or above 150° C. and obtain lower thermal expansion through a reduction of phase formation and thus enhanced thermo-mechanical stability at temperatures up to 240° C. The alloy contains very low amount of Cu (close to zero) and relatively high range of Hf (up to 1.50 wt %) which is very expensive.